Gratings in optical fibers are important structures for optical communications. For example, communications systems using wavelength division multiplexing require gratings to separate the various signals traveling through the optical fibers. Fiber gratings are also used to make sensors. Most fiber gratings are presently fabricated by exposing the core of the fiber to a UV light, having a wavelength around 240 nm, that causes a change in the refractive-index of the fiber's core. However, because the outer polymer coating of most optical fibers is not transparent to 240 nm light, the fiber's outer polymer coating must be removed before exposing the fiber core to the UV light to form the grating. The fiber must then be recoated to prevent damage to the glass fiber and to preserve the mechanical strength of the fiber. This recoating must be done in a timely manner because exposing the surface of a silica fiber to humidity and dirt can permanently weaken the fiber, and the mechanical strength of the once-exposed fiber can remain decreased even after the silica core is subsequently recovered with a coating material. The choice of recoating material is limited by the requirement that the recoating material must adhere well to silica and may need to form a hermetic seal to the fiber surface. Additionally, removing the fiber's polymer coating before UV light exposure and subsequently recoating the fiber with a polymer after the UV light exposure is time consuming and expensive.
Recently, new fiber coatings that are transparent to UV light at 257 nm have been used to coat optical fibers. These new fiber coating make it possible to fabricate fiber gratings without first having to strip the fiber of its coating. However, these fibers with their special coatings have several disadvantages. The transparency of the coating to UV light makes the fibers sensitive to the environment, since undesirable UV light from the environment can now reach the photosensitive fiber core, producing excess optical loss and, in extreme cases, even erasing the fabricated grating. Additionally, these coatings are especially soft and sticky, and can accumulate dust. This dust can adversely affect grating fabrication if the dust absorbs UV light during the fabrication process. Moreover, the polymer coating can also become damaged by excess heat, which can also distort the fiber grating in the fiber core.
An alternative approach for writing gratings in fibers without removing the fiber coating uses the sensitivity of the fiber core to light in the near-UV region of the spectrum, having a wavelength of approximately 330 nm. An advantage to using near-UV light instead of mid-UV light is that the polymer coating of standard optical communication fiber (such as Corning SMF 28, a product of Corning Incorporated, Corning, N.Y.) is somewhat transparent to near-UV light, but is not transparent to mid-UV light having a wavelength of approximately 240 nm. Because standard polymer coatings are transparent to light in this near-UV wavelength region, it becomes possible to fabricate gratings through standard coatings without the use of a special polymer coating. Standard fiber coatings also provide protection to the photosensitive fiber core from mid-UV light having wavelengths in the region of the spectrum where the fiber core has its highest photosensitivity, so that the problem of induced loss and grating erasure by UV light from the environment is reduced. However the problem of degradation of the polymer coating surface from dust and other environmental contaminants remains, because such dust can absorb UV light when the grating is written and distort the light pattern that forms the grating in the fiber core. Special measures to protect the phase mask from contamination with dust and possible exhaust from the polymer during UV exposure may also be required.
What has been needed, and heretofore unavailable is a faster, lower cost method for writing refractive index gratings into optical fiber that avoids the problems of damage or contamination of the coating and subsequent deterioration of the optical path needed to write the grating into the fiber core region. The resulting fiber must have high immunity to erasure or solarization and must retain a significant fraction of its original mechanical strength. The present invention fills this need.